To downsize digital cameras, mobile phones and the like, solid state imaging devices incorporated therein are required to be downsized. The conventional solid state imaging devices are formed such that an image sensor chip with a light receiving section is contained in a package of for example a ceramic to be air-tightly sealed. However, recently a solid state imaging device of CSP (Chip Size Package) type is in widespread use. The CSP type solid state imaging device is formed such that a spacer is firstly introduced to surround an outer periphery of a light receiving section on an image sensor chip, and then a transparent cover glass is attached on the spacer to air-tightly seal the light receiving section. Accordingly, the formed solid state imaging device has a small size approximately same as the image sensor chip which is a bare chip.
As a manufacturing method for the CSP type solid state imaging device, a method in which the packaging is completed on a wafer is proposed (for example, see Japanese Patent Laid-Open Publication No. 2002-231921). In this manufacturing method, at first a wafer on which plural light receiving sections are formed and a transparent substrate (material of the cover glass) on which plural spacers are formed are joined such that each spacer surrounds corresponding light receiving section, and then the wafer and the transparent substrate are diced according to the light receiving sections to make the plurality of solid state imaging devices at one time.
By the way, it is desirable to produce as many image sensor chips as possible from a single wafer of defined size. Therefore, the width of sealing for the light receiving section (width of the spacer) is desired to be narrowed to approximately 100 μm. This sealing width of the CSP type solid state imaging device is much narrower than a sealing width of the conventional package which is about 500 μm to 1 mm. To obtain sufficient sealing performance by the narrow sealing width, an appropriate adhesive is needed to be used. As the adhesive for the CSP type solid state imaging device, there are a heat cure adhesive which is cured by application of heat, an UV cure adhesive which is cured by application of UV light, and a room-temperature-cure adhesive which is cured by passage of time at a room-temperature. However, all of them have both advantages and disadvantages.
The heat cure adhesive has a high glass transition point (Tg) and shows good sealing property and good moisture shielding property at high temperature. Since the heat cure adhesive has low reactivity, current leak or other problems do not occur when being used on wiring patterns of the wafer. In addition, since the heat cure adhesive can be cured in a short time, the productivity becomes pretty good.
However, there is a problem that the heat cure adhesive requires the application of heat to be cured. The cover glass of the solid state imaging device is generally made of a low-α-ray glass to prevent photodiodes of the light receiving portion from being destroyed by a-ray. A coefficient of thermal expansion of the low-α-ray glass is approximately 6.7 ppm/° C., which is approximately twice as large as that of the wafer (generally 2 ppm/° C. to 4 ppm/° C.). Accordingly, when the heat cure adhesive is used for joining of the transparent substrate and the wafer, there is a possibility that a warpage of several millimeters is caused in the joined substrates after the substrates are returned to the room temperature from the high temperature, because of a difference of the coefficient of thermal expansion between the two substrates. The warpage possibly breaks the substrates, or adversely affects post-processes even when the substrates are not broken.
The UV cure adhesive has sealing property and moisture shielding property at high temperature, reactivity and cure speed equal to the heat cure adhesive, and does not require the heating process to be cured. Accordingly, the UV cure adhesive is suitable for joining of components including the low-α-ray glass. However, the UV cure adhesive cannot be used for joining of the spacer and the wafer because the spacer is formed of a material such as silicon, which does not transmit UV light.
Since the room-temperature-cure adhesive does not require the application of heat or light to be cured, it can be used to join the wafer and the spacer. However, the room-temperature-cure adhesive has high reactivity, which possibly causes current leak or other problems on wiring patterns of the wafer. Further, since the room-temperature-cure adhesive has low glass transition point (Tg), sealing property and moisture shielding property at high temperature are insufficient. In addition, the room-temperature-cure adhesive requires long time (for example 16 hours) for being cured, and the productivity becomes worse.
In addition, the solid state imaging device produces heat in image capturing operation. Since the coefficient of thermal expansion is different between the image sensor chip and the cover glass, the junctions between the image sensor chip and the spacer and between the spacer and the cover glass possibly come unstuck because of difference of thermal expansion when being heated.
An object of the present invention is to provide a solid state imaging device and a manufacturing method thereof which prevent above-described problems even when the heat cure adhesive and the UV cure adhesive are used in the joining process.
Another object of the present invention is to provide a solid state imaging device and a manufacturing method thereof which prevent the unstuck of the junctions by the heat in the image capturing operation.